26th August - The plankton collectors

Today we would like to take a look at the plankton sampling. Two instruments are in use for that during our cruise: The plankton net and the multi closing net. Plankton nets are made of fine but tough fabric (our plankton net has a mesh size of 0.5 mm, the multi closing net has a mesh size of 100 µm). They are funnel shaped and are hoisted up through the water column so that the sample is collected in a beaker at the small end of the net. The beaker walls are also made from fabric (or at least some part of it) so that the water can get out of it again. 

The difference between the (simpler) plankton net and the multi closing net is that the latter holds five nets that are lowered in the water column closed and on hoisting the instrument up the five nets can be opened one after the other by a operation unit on board. This allows to get a vertical profile of the community structures in different depths. The mechanism changing the nets is spring loaded so that it must be cocked before deployment.

Once the plankton net or the multi closing net are back on board every nets beaker is emptied into a dish for inspection under the microscope. On emptying the beaker walls are rinsed with filtered seawater to get samples that stick to the wall also into the dish. Under the microscope a whole microcosm of organisms can be seen. We only can show photographs taken with a camera and macro lens and crops of such photos, under the microscope the inividual specimen can be viewed in more detail.
Some bigger species or fish larvae are also found in the net samples like this interesting fellow (of unknown species (about 6 mm long)).

After the inspection under the microscope the samples are fixed and then frozen for later analyses in the lab. Abundance and size distribution of various species under different temperature, salinity and oxygen conditions not only gives clues on planktonic community structures but also hints on how to interpret finds of these species or their remains in sediment cores, helping to build a picture of ocean conditions in the past. Analysing for the content of emerging contaminants will show in how far the environment already is affected by those, considering that plankton is at the base of the marine food web.

(photos: IOW/Ralf Prien, Jassin Petersen - click to enlarge)